In gynecology and obstetrics, two medical parameters are important to assess the condition of the fetus. These two parameters are the fetal beat-to-beat heart rate and uterus (or labor) activity. Simultaneous assessment of the fetal heart rate (FHR) and uterus activity (toco) allows an exact determination of the fetal condition. Monitors measuring and recording both parameters are called "fetal monitors" or "cardiotocographs" (CTG monitors).
To obtain a signal indicative of the fetal heart rate, a so-called fetal scalp electrode may be applied to the fetal skin. These electrodes are usually spiral electrodes which are screwed into the fetal epidermis. See e.g., U.S. Pat. No. 3,827,428. Such direct electrodes allow very accurate measurements due to excellent signal quality. Unfortunately, such internal or direct measurement can only be used after rupture of the membranes. Prior to that point in time (in particular during gestation), indirect methods must be used. These indirect measurements are performed abdominally, e.g. by listening to the fetal heart sound or by measuring the Doppler shift of an ultrasound wave reflected by the moving parts of the fetal heart, particularly the heart walls and the heart valves. Further methods for recording the fetal heart rate employ electrocardiogram electrodes placed on the abdomen of the mother (the abdominal electrocardiogram), the registration of fetal heart sounds with a microphone ("Phono"), the oxygen saturation signal and the like. The present invention relates to all of these methods.
Neither the electrocardiogram, received ultrasound signal, or any other signal indicative of the fetal heart rate is of major diagnostic importance. What is in fact required to assess the fetal condition is the beat-to-beat heart rate. This means that heart beats must be detected in the above signals and the heart rate calculated as the inverse of the time interval between two consecutive beats. The detection of heartbeats and, in particular, the precise determination of the point in time when a beat occurs are difficult tasks. When ultrasound signals are used, many fetal monitors use an autocorrelation technique to detect heartbeats. An autocorrelation signal has a peak that provides an indication of the period of the autocorrelated signal itself. Therefore, it is possible to detect the occurrence of heartbeats even in very noisy signals; however, even the autocorrelation technique does not produce absolutely accurate values for the time interval between two heartbeats, due to the complexity of the received ultrasound signal. That is, even in the case of a silent heart rate, the heart rate trace produced by means of the autocorrelation technique will contain jitter (high-frequency components). This is also true if techniques other than autocorrelation are used; even a direct electrocardiogram obtained by means of a spiral electrode may contain jitter, particularly if the heart rate is high and the sampling rate low (e.g. 200 Hz).
A further problem encountered in fetal monitoring is to ensure that in fact the fetal heart rate and not the maternal heart rate is recorded. There may be several circumstances under which a fetal monitor records the maternal heart rate instead of the fetal one. For example, if an electrocardiogram signal is derived by means of a fetal scalp electrode and the fetus is dead, then the maternal ECG, and therefore the maternal heart rate, may erroneously be measured. The problem is even more serious when the heart rate is derived from an ultrasound signal. If the ultrasound transducer is not precisely focused on the fetal heart, other periodic signals from the mother, e.g. originating from the placenta or the maternal abdominal aorta, may be recorded. This may particularly happen if an autocorrelation technique is used, as this technique tends to lock on a particular signal once detected; i.e., if the algorithm erroneously triggers on the maternal heart rate instead of the fetal heart rate, it will lock on the maternal rate. It is also possible that, due to labor or movement of the fetus, the fetal heart will move away from the ultrasound beam.
The erroneous recording of the maternal heart rate trace instead of the fetal heart trace rate may cause false diagnosis or other dangerous consequences. In order to avoid these situations, a nurse or doctor may measure the maternal rate manually and compare it with an acoustic signal, written recording or optical heart rate indication generated by the fetal monitor. This requires the presence of a clinical person, and is therefore seldom done. Another method is to record the heart rate traces of the fetus and the mother and compare the traces. This method has the same disadvantage, i.e., it requires supervision.
Another problem arises if twins are being monitored--the fetal heart rates may be confused with the maternal heart rate, but also an interchange between the two fetal heart rate traces is possible; e.g., the heart rate trace of the first fetus may be recorded twice instead of the respective heart rate traces of the first and second fetuses.
An object of the present invention is to provide methods and apparatuses for determining the fetal heart rate that generate a reliable warning signal if a heart rate trace originating from the same subject is recorded twice (and thus another heart rate trace of interest is missed). A further object of the present invention is to provide methods and apparatuses, for determining the fetal heart rate, that need no manual supervision. The latter point is of particular importance as it has been observed that, due to a shortage of qualified personnel, manual counter-checks are not made on a regular basis in obstetrical care units.